Buying a generator that's too small means it won't start your air conditioner or well pump when the power goes out. Buying one that's too large wastes money upfront and runs inefficiently. Getting the size right requires understanding the difference between running watts and starting watts — and adding up the loads you actually need.
This guide walks through the complete process of sizing a generator for residential backup, a job site, or a small business.
Running Watts vs. Starting Watts
Every motor-driven appliance has two power ratings: running watts (the continuous power needed during normal operation) and starting watts (the surge of power needed to start the motor). Starting watts can be 2-3 times higher than running watts — and your generator must be able to supply both.
For example, a 1/2 HP well pump might draw 1,000 running watts but require 2,000-3,000 starting watts. If your generator is rated at 2,500 watts, it will struggle or fail to start that pump. You need to size the generator based on the peak starting load, not just the total running load.
Resistive loads like incandescent lights, electric heaters, and most kitchen appliances have little to no startup surge. Inductive loads — motors, air conditioners, refrigerators, well pumps, sump pumps, power tools — all have significant starting surge requirements.
Common Appliance Wattages
Refrigerator: 150-400 running watts, 800-1,200 starting watts. Central AC (3-ton): 3,500 running watts, 9,000+ starting watts. Window AC (10,000 BTU): 1,200 running watts, 3,600 starting watts. Well pump (1/2 HP): 1,000 running watts, 2,100 starting watts. Sump pump (1/3 HP): 800 running watts, 1,300 starting watts.
Electric water heater: 4,000-5,500 watts (no surge). Electric stove burner: 1,200-2,500 watts (no surge). Microwave: 800-1,200 watts. Dishwasher: 1,200-2,400 watts. Washing machine: 500 running watts, 1,500 starting watts. Electric dryer: 5,000 watts.
LED lights: 10-15 watts each. Laptop: 45-100 watts. Desktop computer: 200-300 watts. LED TV: 50-150 watts. Phone charger: 5-25 watts.
How to Calculate Your Generator Size
Step 1: Make a list of everything you want to run during an outage. Divide them into two categories: essential loads (refrigerator, lights, well pump, sump pump, medical equipment) and comfort loads (AC, TV, computers).
Step 2: Find the running watts for each item. Add them all up — this is your running load total.
Step 3: Identify the highest starting-watt item on your list. Add that item's starting watts to the running watts of all the other items. This gives you the minimum starting capacity your generator needs.
Step 4: Add a 20% safety margin to avoid running the generator at 100% load continuously. Running generators near max load stresses the engine and shortens its life.
Example: Refrigerator (300W running) + well pump (1,000W running) + 10 LED lights (150W) + microwave (1,200W) = 2,650 running watts. The well pump has the highest starting surge at 2,100W. Peak demand = 2,100 + 300 + 150 + 1,200 = 3,750W starting. Add 20% = 4,500W minimum. A 5,000W generator covers this load comfortably.
Portable vs. Standby Generators
Portable generators (2,000-12,000 watts) are moved into position when needed, require manual startup, and run on gasoline. They're cost-effective for occasional use but require extension cords or a transfer switch for safe connection to your home's wiring.
Standby generators (7,000-20,000+ watts for residential) are permanently installed, connect to natural gas or propane, and start automatically within seconds of a power outage. They're significantly more expensive ($5,000-$20,000 installed) but require no manual operation.
Never connect a portable generator directly to your home's electrical system without a transfer switch or interlock kit. Backfeeding power through your meter can electrocute utility workers restoring power on the lines outside your home.
Fuel Consumption and Run Time
A typical 5,000-watt gasoline generator consumes about 0.7 gallons per hour at half load. At full load, fuel consumption increases to about 1 gallon per hour. With a 4-5 gallon tank, that's 4-6 hours of runtime — enough for overnight but not for extended outages without refueling.
Natural gas standby generators eliminate the fuel storage problem entirely. They connect to your home's gas line and can run indefinitely as long as the utility gas service is operating — which typically continues during power outages since gas pressure is maintained independently.
Use our Generator Size Calculator to enter your specific loads and get an instant recommendation for the minimum generator size you need, including starting surge requirements.